Demand for wireless services is increasingly rapidly as evident by the tremendous growth in recent years in smart phones. This explosive growth in data traffic and its bandwidth requirements have already saturated the current generation of radio communication networks and will continue to pose a major bandwidth challenge for 3rd Generation Partnership Project (3GPP) systems such as Long-Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS) and High Speed Packet Access (HSPA) and so on.
One of the major challenges faced by operators with the advent of smart phones, tablets and so on is to meet the increased demand for high-speed data services. In order to boost the network capacity, operators are deploying Heterogeneous Networks (HetNets) which are made up of two layers of base stations: a macro-layer consisting of base stations which transmit at high power (e.g., 46 dBm) to provide global coverage and a small-cell layer which consists of pico/femto/relay base stations that transmit at lower power (e.g., 23 dBm) to provide capacity and solve coverage-hole problems. In typical HetNet deployments, several small-cells may be deployed within the coverage area of a macro-cell.
In many countries, spectrum in the higher frequency bands like 3.5 GHz is being freed-up for cellular usage. The relatively poor propagation characteristics associated with carriers in the higher frequency bands make them more suitable for small-cell deployments than macro-cell deployments. In contrast, carriers in the lower frequency bands like 700 MHz, owing to their good indoor as well as outdoor propagation characteristics, are considered more attractive for macro-cell deployments. Thus, the operator deployed HetNet may have the macro layer deployed with one or more carriers in the lower end of the spectrum and the small-cell layer deployed on carriers in the higher end of the spectrum. Depending on the operating load and availability of spectrum, operators may assign either only one carrier at any time to macro and small cell layers or multiple carriers simultaneously to support dual connectivity for increased data rates as well as to ensure robust mobility.
In case of homogenous network, a serving base station may handover the UE to its neighboring base station for load balancing.
In above mentioned scenario, the UEs will be required to perform radio measurements more frequently than when the small-cell layer is not present. Frequent measurements by UE will drain the battery more quickly. In addition, it will reduce throughput of UEs supporting only one RF chain because measurements on non-serving carrier decreases the scheduling opportunities on the serving carrier. To trigger inter-frequency handover at a UE in LTE network, the serving base station executes a process that includes configuring a list of neighbor base station on which the UE will be asked to measure Reference Signal Received Power (RSRP) and/or Reference Signal Received Quality (RSRQ) on cell-specific reference signals, a metric for the received radio signal strength. Configuring appropriate measurement gaps in time to facilitate RSRP/RSRQ measurements on one or more target cells by the UE, and instructing the UE to perform the configured radio measurements on the BS included in the list.
After performing the measurements, the UE sends measurement reports to its serving base station as per the reporting configuration, based on which, the serving base station decides whether the UE should be handed over to one of the target BS reported by the UE. When the UE becomes a candidate for handover and the target BS is ready to accept it, the UE is asked to perform a contention-free Random Access procedure to the target base station to achieve time alignment and obtain an uplink resource grant for subsequent control and data transmissions.